Evidence for ordovician granitoids underneath the po plain: geochronological results on the venice granodiorite

S. Meli, R. Sassi

During the previous decades, several boreholes were drilled by the AGIP company, with the aim of monitoring hydrocarbon occurrence in Italy (AGIP Mineraria, 1977). The “Assunta Borehole”, located within the Venice Lagoon (N 45°26’18’’8, E 12°33’19’’4; sea depth: 15 m), reached the crystalline basement 4711 m below the sea floor; the crosscut sedimentary cover ranges in age from Trias to Quaternary (AGIP Mineraria, 1977). The core recovered from the borehole represents therefore a unique sample of the crystalline basement underneath the Po plain and this unicity makes it worth of special attention. The sampled granitoid has a granodioritic composition, and contains both mafic microgranular enclaves and metamorphic xenoliths of pelitic composition (Meli et al., 1998); their size ranges from few millimeters to 2-3 cm. This report presents U/Pb single grain zircon dating and Rb/Sr whole rock data on the granodiorite and some of the enclaves.

The widespread alteration of feldspars and biotite prevents any internal Rb/Sr isochron to yield reliable results; therefore, whole rock analyses have been tentatively performed on host rock, MME and surmicaceous enclaves. Rb/Sr data clearly demonstrate that the metamorphic enclaves did not isotopically re-equilibrate with the granodiorite, despite their very small size. The couple host rock - MME define a two-point isochron which gives an age of 263±13 Ma, and a (⁸⁷Sr/⁸⁶Sr)_i = 0.71086±0.00087.

A zircon concentrate was gained from the granodiorite applying routine separation techniques. Crystals with very different features (e.g., clearness, presence of inclusions, elongation ratio, roundness, degree of metamictization) were identified. Based upon these characters, four different populations have been distinguished. Due to the small size of the available samples, a statistically significant description of zircon populations was not possible, as less than 70 zircon crystals were extracted. Disregarding highly metamictic zircons, other populations were classified according to the scheme of Pupin (1980): a) clear, elongated, colourless crystals, which mainly belong to S2, S7 and S12 subtypes; b) slightly turbid, elongated, colourless crystals, clustering around S7 and S12 subtypes; c) turbid, not elongated, colourless crystals belonging mainly to S4 and S9 subtypes.

Four single grain conventional U/Pb analyses were performed. Data were corrected for blank and mass spectrometer fractionation (Klötzli, 1997). ²⁰⁶Pb/²⁰⁴Pb was higher than 7500 for all measurements, and common Pb was always lower than 0.6%; therefore, no correction for common Pb has been applied. Two crystals of group a yield perfectly concordant ages of 461±9 and 463±7 Ma (all errors reported are 2s standard errors of the mean), which can be considered identical within the error limits. One crystal of group b gives subconcordant ages (479±40, 470±8, 469±10 Ma for ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²³⁸U respectively). These ages overlap the concordant ones, within the analytical error limits. A crystal coming from group c yields discordant older ages (1435±26, 1237±12, 1001±16 Ma).

Taking into account both concordant and discordant crystals, only a two-point discordia line can be drawn, as group a and b ages greatly overlap within the analytical error. The upper intercept age of 1992±123 Ma has actually no geological meaning at the moment, as it is not possible to detect which mechanism produced the discordance of group c zircon, and whether its discordance would reflect a single stage or a multistage lead loss history. Either CL analyses, which are in progress, or laser probe analyses will solve the problem.

The Th/U ratio recalculated on the basis of ²⁰⁸Pb/²⁰⁶Pb measurements reveals a rough homogeneity among the different populations, ranging from 0.32 to 0.37.

Considering both the age spectra of different zircon populations and their typological/morphological features, the concordant ages of clear, euhedral crystals are thought to mark the emplacement of the granodiorite, therefore pointing to an Upper Ordovician age of intrusion; according to Harland et al. (1989), the concordant ages belong to Caradocian. Inheritance of old cores, possibly combined with Pb loss episodes before the entrapment in the granodioritic melt, can explain the discordance of the turbid crystals of group c.

In the light of the concordant U/Pb ages, MME and granodiorite Rb/Sr whole rock two-point isochron cannot represent the emplacement age, testifying that at least one of them suffered partial reopening after the intrusion of the granodiorite. The recalculated (⁸⁷Sr/⁸⁶Sr)₄₆₃ and (⁸⁷Sr/⁸⁶Sr)₄₆₁ for granodiorite and MME are unreasonably low, thus testifying a post-emplacement reopening of the Rb/Sr system, not only for minerals but also for the whole rocks. Possibly, the marked sericitization of feldspars favoured Rb gain and/or Sr loss, thus leading the Rb/Sr ratio to grow to values higher than those of the unaltered rock.

The U/Pb geochronological age of the Venice granodiorite, together with its petrographic and geochemical characters (Meli et al., 1998), testifies to the occurrence of an unmetamorphosed Upper Ordovician intrusion to the South of Eastern Alps, whose tectonic setting was probably orogenic. Its position constrains southwards the tectono-thermal effects of the Variscan orogeny.